Allows us to uninit cleanly.
This assert was also somewhat pointless as we assert every other
function, so another assert would be triggered long before this
one is.
This commit adds a Vulkan filtering infrastructure for libavfilter.
It attempts to abstract as much as possible of the Vulkan API from filters.
The way the hwcontext and the framework are designed permits for parallel,
non-CPU-blocking filtering throughout, with the exception of up/downloading
and mapping.